The Manchester United striker was taken off on a stretcher in the 14th minutes following a clash of heads with Wesley Hoedt.
Lukaku lay prostrate on the ground for five minutes as he was attended to by the club’s medical staff and continued to receive treatment inside the ground after he had been escorted from the pitch.
‘When you see a player leave the pitch like he did, it is usually two games at least,’ said Mourinho.
United travel to Everton in their next Premier League match before an FA Cup third round tie against Derby County where they will also be without Zlatan Ibrahimovic who will miss the next four weeks with a knee injury.
United have now gone four games without a win in all competitions but Mourinho insists he is happy with the attitude of his players and felt they were unlucky not to be awarded a first half penalty for Maya Yoshida’s handball.
He added: ‘I think it is also a disappointing afternoon for Craig [Pawson, the referee] because he’s a fantastic referee, one of the best in Europe.
‘We missed easy chances but I’m really unhappy with the penalty, not with Pawson’s performance – Today I think Craig had a good performance, it was a big decision to make from Craig.
‘I’m happy with the players’ attitude, the players’ desires and I’m very happy with my place.’
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